The use of benzoxazine compounds in the preparation of thermosetting resins offers a number of advantages, including their relatively long shelf-life, molecular design flexibility, low cost, high glass-transition temperature, high modulus, relatively low viscosities, flame retardancy, low moisture absorption and very low shrinkage. In addition, because their polymerisation is effected via a ring-opening mechanism (as shown for di-functional benzoxazines in scheme 1 below), the generation of troublesome condensation by-products can be avoided.

WO-95/31447-A teaches the preparation of a number of benzoxazine compounds in solvent-less systems. Nevertheless, while benzoxazines have a number of advantages over other thermosetting matrices, their main disadvantage is that they are generally very brittle, and there are no commercially available pure benzoxazine matrices suitable for use in high performance composites. Typically, benzoxazines have a very poor compatibility with commonly used thermoplastic toughening-agents, and this incompatibility leads either to difficulty in dissolving the thermoplastic material into the base resin during mixing or to the gross phase separation of the thermoplastic during cure. Toughening of benzoxazine systems has been limited to the use of rubbers, modified benzoxazine monomers and low performance thermoplastics but these also reduce the beneficial properties of benzoxazines, most notably the modulus and high glass transition temperature.
Several benzoxazine hybrid systems are commercially available (such as the benzoxazine-epoxy hybrid systems available as Araldite® MT resins) but the addition of the co-reactant (epoxy) can negate some of the advantages of benzoxazines. There is currently no available benzoxazine system that retains all of the beneficial properties of neat benzoxazines but that exhibits a toughness suitable for high performance applications.